Energy storage techniques are critical for feasible implantation of sustainable energy, as most sustainable energy sources suffer from intermittent nature, uneven geographically distribution and unstable natural availability. Supercapacitors are a family of energy storage devices capable of storing electric energy converted from sustainable energy such as solar energy and wind energy. They distinguish from another family of energy storage devices, i.e., batteries, by their ability to be fully charged and discharged in seconds. The characteristics of two electrodes in supercapacitors largely dictate the overall performance of supercapacitors. Specifically, developing electrode materials with excellent electrical conductivity, large surface area and exceptional long-term stability (or ultra-long lifetimes) is attracting extensive research effort all over the world to push the performance of supercapacitors to a new height.
The scope of this dissertation covers my past five years of study on design and synthesis of carbonaceous materials, with their applications in supercapacitors. Carbonaceous materials refer to materials composed of and contain carbon. Amorphous carbon shell, graphene, 3D-printed graphene aerogel, 3D hierarchical porous carbon foams and conducting polymers are included in the dissertation. They share common advantages such as they are electrically conductive, economically viable, easy to be processed into versatile architectures with ultra-large surface area (>1000 m2 g-1).
This dissertation will present my research works associated with carbonaceous materials for supercapacitors carried out in the past five years. Chapter one sets the background for energy storage and introduces key concepts of supercapacitors. Chapter two and chapter three present hierarchical porous carbon foams derived from chitosan, with modifications by incorporation of potassium carbonate (chapter two) and silica hard templates (chapter three), that exhibited superior capacitive performance to other state-of-the-art carbon electrodes. Chapter four introduces a 3D graphene aerogel wood-pile lattice with orderly distributed macropores. This graphene aerogel structure was fabricated by direct ink writing (one of 3D printing techniques). Supercapacitors using these 3D-printed graphene aerogel electrodes with thickness on the order of millimeters display exceptional capacitive retention (approximately 90% from 0.5 to 10 A g-1) and power density (>4 kW kg-1) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. A three-step ion-intercalation method that can further boost the capacitance of the 3D printed graphene aerogel by two times is discussed in chapter five. Chapter six and seven deal with increasing cycling stability of typical conjugated polymers with capacitive activity, e.g., polyaniline and polypyrrole by carbonaceous materials. Finally in chapter eight, an outlook on carbonaceous materials will be given. Specifically, current challenges and opportunities for developing hierarchical porous carbons as high-performance supercapacitor electrodes are discussed.
